The present disclosure relates generally to the fabrication of semiconductor devices, and more particularly, to a system for removing a spacer used to define the lightly doped drain (LDD) regions in Metal Oxide Semiconductor Field Effect Transistor (MOSFET) fabrication.
Semiconductor device geometries have dramatically decreased in size since such devices were first introduced several decades ago. Since then, integrated circuits have generally followed the two year/half-size rule (often called Moore's Law), which means that the number of devices on a chip doubles every two years. Today's fabrication plants are routinely producing devices having 0.35 μm and even 90 nm feature sizes or smaller. As geometries shrink, semiconductor manufacturing methods often must be improved.
Traditional methods for fabricating the MOSFETs used in integrated circuit structures are becoming inadequate as device size shrinks. Conventional MOSFET fabrication utilizes a technique of building material spacers to help control and define the implantation of dopants in the source and drain regions of the MOSFET. One way to control the implantation of dopants is by using an LDD region in a semiconductor substrate between the channel region (e.g., the region of the substrate beneath a gate electrode and a gate oxide) and the more heavily doped source and drain regions. This LDD region between the channel and the more heavily doped conventional drain region reduces the electric field thereby mitigating short-channel effects, reducing hot-carrier generation, and increasing the junction breakdown voltage. The LDD region provides a gradual transition from the drain and/or source to the gate region. This transition area disburses any abrupt voltage changes and reduces the maximum electric field strength. A discussion of the LDD region may be found in S. Wolf, Silicon Processing for the VLSI Era 348 (Vol. 2, Lattice Press 1990).
Spacers are often used in the fabrication of LDD regions to facilitate the different levels of doping for the drain/source regions and the LDD regions. The LDD region can be controlled by the lateral spacer dimension and the thermal drive cycle, and can be independent from the source and drain implant depth. However, removing the spacer is critical because removal can damage adjacent structures, such as the gate and the underlying silicon substrate. This difficulty is exacerbated during the LDD formation process which can produce a hard polymer layer on top of the spacer, making its removal more difficult.
Other difficulties must also be considered. Layer thickness decreases and sensitivity to heat exposure (the thermal budget) needed to provide annealing and activation of dopants become critical as device geometries decrease. Also, transient enhanced diffusion (TED) can cause the LDD region to undesirably extend in both vertical and horizontal directions during the formation of such items as sidewall spacers. As device geometries shrink, the harmful effects of TED have become a greater problem, prompting efforts to eliminate any spacer made during the semiconductor fabrication.